A head-up display apparatus (hereinafter “HUD apparatus”) providing a virtual image of a display image produces by a laser beam (light beam) by diffusing the laser beam with a screen toward a projection plane has been conventionally known.
For example, a HUD apparatus disclosed in a patent document 1 diffuses a laser beam, which is projected from a projector to a screen, with a plurality of optical elements arranged in a grid pattern. The laser beam diffused and projected on a projection plane is visible as a virtual image of a display image for a viewer in an interior of a vehicle.
However, when a high coherence laser beam is projected toward and diffused with the optical elements having a regular pattern of the grid arrangement, the virtual image having luminance irregularity may be produced (i.e., not uniformly illuminated).